<p>Placemarks can contain arbitrarily defined custom data. First, you must define
  a <code>Schema</code> that contains <code>SimpleField</code> elements. Placemarks
  can then implement this schema in an <code>ExtendedData</code> section
  using <code>SchemaData</code> and <code>SimpleData</code> elements. The data
  can then be used as substitution variables in the balloon HTML.</p>
